[Techtalk] Alsa/Pulse oddity

[David Sumbler]

I recently installed Ubuntu 20.04 on my computer to replace 18.04.
 But I still have 18.04, which is on a different partition, and the 2
versions share another partition as /home.

Sound works fine on 18.04, and is mostly OK on 20.04 except that my
headphones don't work.  In alsamixer I have a control for headphones,
but not in pavucontrol, and simlarly my headphones don't show up in
System Settings - Sound either.

If I run 'aplay -l' under either OS version I get:

**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC887-VD Analog [ALC887-VD
Analog]
Subdevices: 0/1
Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 1: ALC887-VD Digital [ALC887-VD
Digital]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 2: NVidia [HDA NVidia], device 3: HDMI 0 [HDMI 0]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 2: NVidia [HDA NVidia], device 7: HDMI 0 [HDMI 0]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 2: NVidia [HDA NVidia], device 8: HDMI 0 [HDMI 0]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 2: NVidia [HDA NVidia], device 9: HDMI 0 [HDMI 0]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 3: Speaker [USB Speaker], device 0: USB Audio [USB Audio]
Subdevices: 1/1
Subdevice #0: subdevice #0

The first of these items seems to be my headphones, because if I run
'aplay -D plughw:0,0 /usr/share/sounds/alsa/Front_Center.wav' 
under 18.04 the file plays through my headphones.  But in 20.04 I just
get an error message:

aplay: main:830: audio open error: Device or resource busy

And strangely, if I open Audacity and go to Edit-Preferences-Devices
and select ALSA as Host (the only option, in fact), then in the
Playback Device drop down list, all the devices shown by 'aplay -l' are
listed except for the first - my headphones!

Can anyone suggest why alsa seems to know about my headphones, up to a
point, but for some reason they are not appearing elsewhere?  The
"Device or resource busy" message seems to be significant, but I don't
know what to make of it.

David